fix coverage

tox.ini

@@ -47,18 +47,18 @@ commands = pre-commit run --all-files --show-diff-on-failure
 
 
 [testenv:coverage]
-description = [run locally after tests]: combine coverage data and create reports
+description = [run locally after tests]: combine coverage data and create report;
+              generates a diff coverage against origin/master (can be changed by setting DIFF_AGAINST env var)
 deps = {[testenv]deps}
        coverage >= 4.4.1, < 5
-       diff_cover
 skip_install = True
 setenv = COVERAGE_FILE={toxworkdir}/.coverage
 changedir = {toxinidir}
 commands = coverage erase
            coverage combine
            coverage report -m
-           coverage xml
-           diff-cover --compare-branch upstream/master {toxworkdir}/coverage.xml
+           coverage xml -o {toxworkdir}/coverage.xml
+           diff-cover --compare-branch {env:DIFF_AGAINST:origin/master} {toxworkdir}/coverage.xml
 
 [testenv:codecov]
 description = [only run on CI]: upload coverage data to codecov (depends on coverage running first)